Directors NI - Alternative Method

I am trying to use https://www.gov.uk/guidance/work-out-directors-national-insurance-contributions to check Director and ER conts in our test payroll systems. I seem to have the director calcualting as per this HMRC calc. However, I have an employee where there is a slight £0.50 reduction due. HMRC calc shows '£0.00' rather than £-.50? Does anyone know, does this calculator show a -ve value is a reduction/refund is due?

Hi,
A negative value on your payroll software does not necessarily mean a refund is due.  
Rather, it is more likely reporting a reduction to the in-year liabiity for National Insurance.  
Consequently, you would reduce your payments to us where there is an ongoing employer liability.  
However, this is most commonly seen for directors, who are entitled to a different calculation method.
The question seems to indicate this is for an ordinary employee, this would not normally be the case, unless perhaps an incorrect NIC category letter was being used.
It is probably best that you speak with us so we can get the full picture.  
Please call the Employer Help Line on 03002003200. 
Thank you.